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Overview :
The inoculum effect refers to an
increase in the MIC for an isolate to an antibiotic associated with a higher
inoculum into the liquid test media. This is classically seen with a
Staphyloccus species tested against a beta-lactam antibiotic, but it has been
reported with many bacteria against many antibiotics.
Criteria - both of the
following:
(1) An increase in the inoculum size >= 0.5 LOG10 is associated with
(2) A significant increase in the MIC (at least 2 dilutions, or a 4-fold
increase in the antibiotic concentration).
where:
• A 0.5 LOG10 increase is about
3.2 times.
Theories for its occurrence:
(1) inactivation by
drug-targeting enzymes
(2) selection for a
drug-resistant strain
Limitations:
• It is essential to control the inoculum size, incubation time,
incubation period, reagent vessel and other testing factors.
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